1. Field of the Invention
This invention relates to a process for calcining magnesium sulfate in the presence of a reducing agent to convert it into magnesium oxide. More particularly, it is concerned with a process for the production of magnesium oxide at low temperatures and in high conversions by calcining magnesium sulfate in the presence of a specific reducing agent.
2. Description of the Prior Art
It is known to recover a sulfite gas contained in flue gas as magnesium sulfite by treating the flue gas with a slurry of magnesium hydroxide for desulfurization of the flue gas. This magnesium sulfite is pyrolyzed and re-used as magnesium oxide. The magnesium sulfite obtained at the step of desulfurization of flue gas contains magnesium sulfate as a by-product. In order to increase the economic efficiency of the process, therefore, it is necessary to convert the magnesium sulfate into magnesium oxide by pyrolysis.
The pyrolysis temperature of magnesium sulfate, however, is as high as about 1,000.degree. C., which is quite high as compared with that of magnesium sulfite; i.e., about 600.degree. C. At such high temperatures, magnesium oxide is excessively heated and cannot be re-used.
In order to solve the above problem, magnesium sulfite containing magnesium sulfate, obtained at the step of desulfurization of flue gas has hitherto been calcined at temperatures as high as 800.degree. to 900.degree. C. in the presence of petroleum coke as a reducing agent which is added to decrease the heat decomposition temperature of magnesium sulfate. This procedure, however, suffers from the disadvantage that the conversion of magnesium sulfate at low temperature zones is low, and it is still unsatisfactory. For example, a bench test shows that the conversion is less than 60% at calcining temperatures of lower than 800.degree. C. In addition, there is a distribution of temperature in a furnace. Therefore, even though the operating temperature is set to 900.degree. C., there are several zones in the furnace where the temperature is lower than 800.degree. C. Thus, if it is intended to completely heat magnesium sulfate up to 800.degree. C. at which the conversion of magnesium sulfate to be calcined is more than 60%, it is necessary to lengthen the residence time of the magnesium sulfate in the furnace. This leads to a reduction in the calcining efficiency.